326 Professor John Milne [Feb. 12, 



WEEKLY EVENING MEETING, 

 Friday, February 12, 1897. 



George Matthet. Esq. F.E.S. F.C.S. Vice-President, in tlie Chair. 



Professor John Milne, F.R.S. F.G.S. 



Becent Advances in Seismology. 



As an introduction to the discourse for this evening, I feel it my 

 duty to call attention to the broad meaning which it now seems 

 necessary to apply to the word Seismology. Only a few years ago 

 the occupation of the seismologist was strictly confined to the study 

 of sudden movements which from time to time take place in the 

 crust of our earth. These movements, although sometimes violent, 

 were to him transient phenomena which seldom continued longer 

 than a few seconds, or at the most one or two minutes. Recent 

 investigations have shown that the same disturbances are preceded 

 by minute tremors which, under certain conditions, may last many 

 minutes, whilst after all movement to which we are sensible has 

 ceased, the ground may palpitate for many hours. Another set of 

 phenomena to which attention is now directed, are the earthquakes 

 which are repeated many times per year in every country in the 

 world, which by our unaided senses are passed by unnoticed. In 

 short, the unfelt evidences of seismicity are much more general than 

 those which are accompanied by destruction and alarm, and a new 

 seismology has been discovered which is at least as important as 

 the old. 



Now that we are assured that the greater number of earthquakes 

 are but intermittent accelerations in the more general movements of 

 rock folding and rock crushing, to separate the announcements that 

 these mighty changes are in operation from the changes themselves, 

 is to separate an infant from its parent, an effect from its cause. 

 Besides these legitimate relations of earthquakes, the practical 

 seismologist finds that he often records movements of a quasi-seismic 

 origin, together with others like diurnal waves, and tremors which 

 find an explanation in causes external to the surface of our earth. 

 These latter are at present without a home, and although they are 

 non-seismic, in many instances at least, they represent actual move- 

 ment in the ground, and seismology finds itself in the position of 

 foster-mother to strange children. These various movements which 

 take place within and on the surface of the earth, the study of which 

 may, until we find a more suitable word, be embraced under the term 

 seismology, are indicated in the following table : — 



